Modelling Geothermal Systems
The Northern Lights Over Reykjavik, Iceland 27 Feb 2021 - Photo by Ben Smith
by Benjamin D. Smith
About Ben Smith: Ben is a geologist pursuing a Master of Science Degree in Sustainable Energy at Reykjavik University in Iceland
In my graduate studies at Reykjavík University I have been fortunate to gain experience using the Leapfrog 3D subsurface geothermal modelling software. With this modelling software you can make intelligent decisions including well targeting, numerical analysis, testing well trajectories, anticipate permeability, simulate flow, etc. Modelling is an essential decision-making tool. The benefits of geothermal system modelling extend far beyond the acquisition of heat alone, geothermal fluids are an increasingly important source of valuable metals such as lithium. With the growing demand for lithium to produce batteries for electric vehicles, geothermal system modelling will grow in importance.
Let’s take a quick tour of what the geothermal modelling process looks like.
Geological Exploration
The process is initiated as geologists look for potential sites with high production potential. This normally starts with non-invasive exploration techniques such as finding surface manifestations, conducting resistivity tests, performing chemical analysis, and more. Before drilling any holes, an initial conceptual model the geothermal area is created. This model is continuously improved throughout the process as new data is obtained.
The Conceptual Model
A conceptual model lays the foundation for any geothermal project. Its purpose is to create a representation of the actual system in a simplified manner. This model evolves as more information about the system is discovered. The initial model could be as simple as a drawing on a napkin, yet, with more data comes more complexity. The figure above shows a digital conceptual model. This model shows that we have a good idea of the geology of the area along with data of some faults. The model is then updated as wells are drilled. The image below displays a temperature gradient created using data from drilled wells to update the conceptual model.
Simulation Model
The conceptual model is then used to create a numerical model to simulate conditions and make predictions about the future. A major use of these models is to predict future effects of production or reinjection. In this example, a grid was created using Leapfrog for use with their TOUGH2 numerical simulator. Leapfrog creates a grid type model made of data blocks to be exported. The size of the blocks dictates the resolution you will receive form the model, if there is an uninteresting section, a larger block size will be given, with an area with high flow rates or fractures, a smaller block size will be given.
The figure below shows what a model in TOUGH2 might look like. The different colors represent different rock types. This model simulates hot geothermal fluids migrating upwards in a geothermal setting. The simulation solves various equations such as Darcy’s flow, fluid thermodynamics, and mass transfer. Before a model can be used to predict future production, it must first be calibrated to represent current conditions. If the model is not properly calibrated, it will not provide accurate future predictions. At this point, critical thinking skills come into play to fix any modelling problems. Potential problems might include faults that are not accounted for or maybe the feed zones are in different areas. There could be a wide range of problems to discover and address. The exciting challenge with modelling is solving these types of problems.
Applications
The ability to model fluid rock interaction has many uses in the real world. Being able to accurately predict the effects of production and reinjection of fluid into a reservoir can make or break a power plant project as well as associated metal extraction aspirations.
Programs such as TOUGH2 also have the ability to model other simulations involving fluid rock interaction such as, nuclear waste disposal, carbon sequestration projects, and contaminant plume tracking.
I graduate this Summer and look forward to putting these skills to use.
Contact: bendsmith917@gmail.com
